howdy ya'll, long time lurker, first time poster [finally something in my lane]. so, at the altitudes discussed, water does indeed spontaneously boil. the amount of force keeping water in solution goes down as air pressure drops at altitude. The boiling point eventually drops to 98.6 degrees F, and whammo....your blood boils. the medical term for this unfortunate condition is "ebullism". The exact altitude at which it happens depends on barometric pressure and temperature, but in general we just say that it occurs around 63,000'. (called the "Armstrong Line") Consequently, folks wishing to exit at that altitude need to be wearing some kind of pressure suit in order to prevent their inside parts from becoming their outside parts. The world record jumper is Colonel Joseph Kittinger. I'm sure this isn't news to most of you, but in 1960 he jumped at 102,000'. (if you're not familiar with the story of his life, you should look into it. Not only is he credited with this heroic feat, he's also a combat pilot from Vietnam with two Silver Stars and a Prisoner of War Medal which he earned during his stay at the Hanoi Hilton) Now, on to the "speed of sound" issue. The rate that sound travels depends on the medium that it's traveling through. In dry air, at room temperature it's 768mph. In water sound travels several times faster (~3300mph) and in a vacuum, sound doesn't travel at all. If "Fearless Felix" is able to exit at 120,000 will he "break the sound barrier"? Well, maybe, but not in the traditional way we think of it. He'll continue to accelerate (at a rate less than the 9.8m/s/s that Newton's Apple fell) until he reaches a terminal velocity for given conditions. The force required to push his body through the sound barrier isn't linear and I suspect that as he approaches that limit he'll experience a spike in relative drag that will serve to buffer him from supersonic speeds. If I'm wrong, the transition into supersonic flight is likely to be very rough for him. I suspect he'll become extremely unstable at that point. Traditional flight positions would be very dangerous. All-in-all, it's a fascinating project that I'd love to be working on. Very jealous! -trip.